Cleaning 4 inch trap, what to use?

I think the trap on my mom's house's main drain line is clogged. It's a 4" J(?) trap just before the drain exits the house. I got the top off, but now I'm not sure what to clean the trap with! My 1/4" snake seems too small and my 3/8" drum snake seems too rigid to make the bend.

BTW this trap looks like a P trap except the clean out is on top. It's cast-iron, probably from the 1930s or before.

Also, I'm probably in trouble because I had to cut the cap off & I doubt I could buy a match. Could I get away with a test plug?

If you are SURE it is just the trap, then a wet/dry vacuum will suck the water and muck out of the trap. Usually, those traps have cleanouts on both sides because it is difficult, if not impossible, to "snake" through it.

I routinely push a 11/16" cable with a 3" round blade through 4" house traps using my K-7500.
It does a great job of scraping the sides of the trap clean and I usually do this from the outlet side so the water backed up on the house side can wash most of the debris out.

After that I use a wet vac to empty any remaining debris out of the trap.

If the Mass Plumbing Code allows removal of the trap I would hire a plumber to do so.